1. Field of the Invention
The present invention relates to electrical fixtures and wiring devices, and more particularly to systems and methods for reducing electrically-caused fires in fixtures and wiring devices due to excessive heating at wiring connections.
2. Background and Related Art
Many building fires occur because of faults in series with a power outlet, light switch or other fixture. For example, loose connections cause points of excessive heating under normal use. If not detected, this localized heating leads to fires by igniting wire insulation, fixture materials, framing, or other nearby flammable objects. The fires caused by the excessive localized heating result in property loss, injury and even death.
These points of excessive heating are most commonly found at the wiring connection to the fixture. The excessive heating happens for one of several different reasons that include: an installer who neglects to tighten or fully tighten screws on the supply wires; regular use that loosens supply wire connections; supply wire connection materials that become oxidized; and/or an unqualified installer who connects supply wires improperly. Any of these conditions/causes leads to unexpected electrical resistance at the connection point, and electrical current flowing through the higher-than-expected electrical resistance causes the overheating and fires described above.
FIG. 1 illustrates the effect of a series fault. The left-most screw of the illustrated plug receptacle was not tightened sufficiently and overheated, melting the insulation on the supply wire. Furthermore, the screw oxidized, increasing the excessive heating and leading to melted plastic and a destructive fire.
Attempts have been made to create electrical wiring devices, such as plug receptacles and outlets, that detect heating and that discontinue electrical power draw to eliminate the overheating condition. Currently-available devices and methods rely on bimetallic thermal sensors acting as a switch to cause a disconnect in the electrical current. When the electrical current is interrupted, the power delivered to the high-resistance connection stops, along with the heating generated by the power lost at the connection. Such devices have proved difficult to implement. For example, typical bimetallic thermal sensors/switches, such as one of brass and invar, have a switching threshold of approximately 200 degrees Fahrenheit. While most plastic household wiring insulation and outlet housings do not melt until temperatures reach or exceed approximately 300 degrees Fahrenheit, operation approaching 200 degrees Fahrenheit has a high probability of causing distortion of the materials. Additionally, it is possible for heat to exceed 200 degrees Fahrenheit in one location of the device before the bimetallic switch itself is heated sufficiently to cause thermal switching. Because of the bulk of typical bimetallic switches, it is difficult to locate such switches close to the locations of potential heating, and thus such bimetallic switches fail to adequately protect against over-heating even when they have a lower temperature threshold for switching.
Some approaches have tried to address differences in heating location by using multiple bimetallic switches or using heat-conductive materials in the devices. Such attempts lead to higher manufacturing costs and also fail to address the fact that the 200-degree threshold of the bimetallic thermal switching, while preventive of fires, fails to prevent material distortion with its attendant risks and difficulties.
Current circuit breakers and fuses are unable to detect points of excessive heating, because they measure electric current rather than temperature. The electric current flowing through a point of excessive heating is typically within the range of normal current flow of circuit breakers and fuses. Arc Fault Circuit Interrupters (AFCI) are a type of circuit breaker technology that is capable of detecting parallel faults, or faults between line and neutral that are in parallel with the outlet or device. AFCI devices do not provide protection against series faults that lead to glowing connections (overheating) and subsequent fires.